Electro-optic displays including redox compounds

ABSTRACT

An electro-optic display having a viewing surface through which a user views the display, a bistable, non-electrochromic electro-optic medium, and at least one electrode arranged to apply an electric field to the electro-optic medium, the display further comprising at least 10 micromoles per square meter of the viewing surface of at least one compound having an oxidation potential more negative that about 150 mV with respect to a standard hydrogen electrode, as measured at pH 8, where the compound

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/639,258 filed on Jun. 30, 2017, which is a divisional application ofU.S. patent application Ser. No. 14/152,067 filed on Jan. 10, 2014, nowU.S. Pat. No. 9,726,957 issued Aug. 8, 2017, where the Ser. No.14/152,067 application, itself, claims priority to United StatedProvisional Application No. 61/750,980 filed on Jan. 10, 2013.

The entire contents of these patents and copending application, and ofall other U.S. patents and published and copending applicationsmentioned below, are herein incorporated by reference.

BACKGROUND OF INVENTION

This invention relates to electrode structures. These electrodestructures are especially intended for use in electro-optic displays butmay also find use in other applications.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin U.S. Pat. No. 7,170,670 that some particle-based electrophoreticdisplays capable of gray scale are stable not only in their extremeblack and white states but also in their intermediate gray states, andthe same is true of some other types of electro-optic displays. Thistype of display is properly called “multi-stable” rather than bistable,although for convenience the term “bistable” may be used herein to coverboth bistable and multi-stable displays.

The term “electro-optic”, as applied to a material or a display, is usedherein in its conventional meaning in the imaging art to refer to amaterial having first and second display states differing in at leastone optical property, the material being changed from its first to itssecond display state by application of an electric field to thematerial. Although the optical property is typically color perceptibleto the human eye, it may be another optical property, such as opticaltransmission, reflectance, luminescence or, in the case of displaysintended for machine reading, pseudo-color in the sense of a change inreflectance of electromagnetic wavelengths outside the visible range.

Some electro-optic media are solid in the sense that the materials havesolid external surfaces, although the media may, and often do, haveinternal liquid- or gas-filled spaces. Displays using solidelectro-optic media may hereinafter for convenience be referred to as“solid electrophoretic displays”.

Several types of electro-optic displays are known. One type ofelectro-optic display is a rotating bichromal member type as described,for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761;6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791(although this type of display is often referred to as a “rotatingbichromal ball” display, the term “rotating bichromal member” ispreferred as more accurate since in some of the patents mentioned abovethe rotating members are not spherical). Such a display uses a largenumber of small bodies (typically spherical or cylindrical) which havetwo or more sections with differing optical characteristics, and aninternal dipole. These bodies are suspended within liquid-filledvacuoles within a matrix, the vacuoles being filled with liquid so thatthe bodies are free to rotate. The appearance of the display is changedby applying an electric field thereto, thus rotating the bodies tovarious positions and varying which of the sections of the bodies isseen through a viewing surface. This type of electro-optic medium istypically bistable.

Another type of electro-optic display uses an electrochromic medium, forexample an electrochromic medium in the form of a nanochromic filmcomprising an electrode formed at least in part from a semi-conductingmetal oxide and a plurality of dye molecules capable of reversible colorchange attached to the electrode; see, for example O'Regan, B., et al.,Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24(March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845.Nanochromic films of this type are also described, for example, in U.S.Pat. Nos. 6,301,038; 6,870,657; and 6,950,220. This type of medium isalso typically bistable.

Another type of electro-optic display is an electro-wetting displaydeveloped by Philips and described in Hayes, R. A., et al., “Video-SpeedElectronic Paper Based on Electrowetting”, Nature, 425, 383-385 (2003).It is shown in U.S. Pat. No. 7,420,549 that such electro-wettingdisplays can be made bistable.

One type of electro-optic display, which has been the subject of intenseresearch and development for a number of years, is the particle-basedelectrophoretic display, in which a plurality of charged particles movethrough a fluid under the influence of an electric field.Electrophoretic displays can have attributes of good brightness andcontrast, wide viewing angles, state bistability, and low powerconsumption when compared with liquid crystal displays. Nevertheless,problems with the long-term image quality of these displays haveprevented their widespread usage. For example, particles that make upelectrophoretic displays tend to settle, resulting in inadequateservice-life for these displays.

As noted above, electrophoretic media require the presence of a fluid.In most prior art electrophoretic media, this fluid is a liquid, butelectrophoretic media can be produced using gaseous fluids; see, forexample, Kitamura, T., et al., “Electrical toner movement for electronicpaper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y.,et al., “Toner display using insulative particles chargedtriboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Pat.Nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic mediaappear to be susceptible to the same types of problems due to particlesettling as liquid-based electrophoretic media, when the media are usedin an orientation which permits such settling, for example in a signwhere the medium is disposed in a vertical plane. Indeed, particlesettling appears to be a more serious problem in gas-basedelectrophoretic media than in liquid-based ones, since the lowerviscosity of gaseous suspending fluids as compared with liquid onesallows more rapid settling of the electrophoretic particles.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporationdescribe various technologies used in encapsulated electrophoretic andother electro-optic media. Such encapsulated media comprise numeroussmall capsules, each of which itself comprises an internal phasecontaining electrophoretically-mobile particles in a fluid medium, and acapsule wall surrounding the internal phase. Typically, the capsules arethemselves held within a polymeric binder to form a coherent layerpositioned between two electrodes. The technologies described in thethese patents and applications include:

-   -   (a) Electrophoretic particles, fluids and fluid additives; see        for example U.S. Pat. Nos. 7,002,728 and 7,679,814;    -   (b) Capsules, binders and encapsulation processes; see for        example U.S. Pat. Nos. 6,922,276 and 7,411,719;    -   (c) Films and sub-assemblies containing electro-optic materials;        see for example U.S. Pat. Nos. 6,982,178 and 7,839,564;    -   (d) Backplanes, adhesive layers and other auxiliary layers and        methods used in displays; see for example U.S. Pat. Nos.        D485,294; 6,124,851; 6,130,773; 6,177,921; 6,232,950; 6,252,564;        6,312,304; 6,312,971; 6,376,828; 6,392,786; 6,413,790;        6,422,687; 6,445,374; 6,480,182; 6,498,114; 6,506,438;        6,518,949; 6,521,489; 6,535,197; 6,545,291; 6,639,578;        6,657,772; 6,664,944; 6,680,725; 6,683,333; 6,724,519;        6,750,473; 6,816,147; 6,819,471; 6,825,068; 6,831,769;        6,842,167; 6,842,279; 6,842,657; 6,865,010; 6,967,640;        6,980,196; 7,012,735; 7,030,412; 7,075,703; 7,106,296;        7,110,163; 7,116,318; 7,148,128; 7,167,155; 7,173,752;        7,176,880; 7,190,008; 7,206,119; 7,223,672; 7,230,751;        7,256,766; 7,259,744; 7,280,094; 7,327,511; 7,349,148;        7,352,353; 7,365,394; 7,365,733; 7,382,363; 7,388,572;        7,442,587; 7,492,497; 7,535,624; 7,551,346; 7,554,712;        7,583,427; 7,598,173; 7,605,799; 7,636,191; 7,649,674;        7,667,886; 7,672,040; 7,688,497; 7,733, 335; 7,785,988;        7,843,626; 7,859,637; 7,893,435; 7,898,717; 7,957,053;        7,986,450; 8,009,344; 8,027,081; 8,049,947; 8,077,141;        8,089,453; 8,208,193; and 8,373,211; and U.S. Patent        Applications Publication Nos. 2002/0060321; 2004/0105036;        2005/0122306; 2005/0122563; 2007/0052757; 2007/0097489;        2007/0109219; 2007/0211002; 2009/0122389; 2009/0315044;        2010/0265239; 2011/0026101; 2011/0140744; 2011/0187683;        2011/0187689; 2011/0286082; 2011/0286086; 2011/0292319;        2011/0292493; 2011/0292494; 2011/0297309; 2011/0310459; and        2012/0182599; and International Application Publication No. WO        00/38000; European Patents Nos. 1,099,207 B1 and 1,145,072 B1;    -   (e) Color formation and color adjustment; see for example U.S.        Pat. No. 7,075,502 and U.S. Patent Application Publication No.        2007/0109219;    -   (f) Methods for driving displays; see for example U.S. Pat. Nos.        7,012,600; 7,119,772; and 7,453,445;    -   (g) Applications of displays; see for example U.S. Pat. Nos.        7,312,784 and 8,009,348; and    -   (h) Non-electrophoretic displays, as described in U.S. Pat. Nos.        6,241,921; 6,950,220; 7,420,549 and 8,319,759; and U.S. Patent        Application Publication No. 2012/0293858.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes ofthe present application, such polymer-dispersed electrophoretic mediaare regarded as sub-species of encapsulated electrophoretic media.

A related type of electrophoretic display is a so-called “microcellelectrophoretic display”. In a microcell electrophoretic display, thecharged particles and the fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. See, forexample, U.S. Pat. Nos. 6,672,921 and 6,788,449, both assigned to SipixImaging, Inc.

Although electrophoretic media are often opaque (since, for example, inmany electrophoretic media, the particles substantially blocktransmission of visible light through the display) and operate in areflective mode, many electrophoretic displays can be made to operate ina so-called “shutter mode” in which one display state is substantiallyopaque and one is light-transmissive. See, for example, U.S. Pat. Nos.5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and6,184,856. Dielectrophoretic displays, which are similar toelectrophoretic displays but rely upon variations in electric fieldstrength, can operate in a similar mode; see U.S. Pat. No. 4,418,346.Other types of electro-optic displays may also be capable of operatingin shutter mode. Electro-optic media operating in shutter mode may beuseful in multi-layer structures for full color displays; in suchstructures, at least one layer adjacent the viewing surface of thedisplay operates in shutter mode to expose or conceal a second layermore distant from the viewing surface.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word “printing” is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes;electrophoretic deposition (See U.S. Pat. No. 7,339,715); and othersimilar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively.

An electro-optic display normally comprises a layer of electro-opticmaterial and at least two other layers disposed on opposed sides of theelectro-optic material, one of these two layers being an electrodelayer. In most such displays both the layers are electrode layers, andone or both of the electrode layers are patterned to define the pixelsof the display. For example, one electrode layer may be patterned intoelongate row electrodes and the other into elongate column electrodesrunning at right angles to the row electrodes, the pixels being definedby the intersections of the row and column electrodes. Alternatively,and more commonly, one electrode layer has the form of a singlecontinuous electrode and the other electrode layer is patterned into amatrix of pixel electrodes, each of which defines one pixel of thedisplay. In another type of electro-optic display, which is intended foruse with a stylus, print head or similar movable electrode separate fromthe display, only one of the layers adjacent the electro-optic layercomprises an electrode, the layer on the opposed side of theelectro-optic layer typically being a protective layer intended toprevent the movable electrode damaging the electro-optic layer.

The manufacture of a three-layer electro-optic display normally involvesat least one lamination operation. For example, in several of theaforementioned MIT and E Ink patents and applications, there isdescribed a process for manufacturing an encapsulated electrophoreticdisplay in which an encapsulated electrophoretic medium comprisingcapsules in a binder is coated on to a flexible substrate comprisingindium-tin-oxide (ITO) or a similar conductive coating (which acts asone electrode of the final display) on a plastic film, thecapsules/binder coating being dried to form a coherent layer of theelectrophoretic medium firmly adhered to the substrate. Separately, abackplane, containing an array of pixel electrodes and an appropriatearrangement of conductors to connect the pixel electrodes to drivecircuitry, is prepared. To form the final display, the substrate havingthe capsule/binder layer thereon is laminated to the backplane using alamination adhesive. (A very similar process can be used to prepare anelectrophoretic display usable with a stylus or similar movableelectrode by replacing the backplane with a simple protective layer,such as a plastic film, over which the stylus or other movable electrodecan slide.) In one preferred form of such a process, the backplane isitself flexible and is prepared by printing the pixel electrodes andconductors on a plastic film or other flexible substrate. The obviouslamination technique for mass production of displays by this process isroll lamination using a lamination adhesive. Similar manufacturingtechniques can be used with other types of electro-optic displays. Forexample, a microcell electrophoretic medium or a rotating bichromalmember medium may be laminated to a backplane in substantially the samemanner as an encapsulated electrophoretic medium.

As discussed in the aforementioned U.S. Pat. No. 6,982,178, (see column3, lines 63 to column 5, line 46) many of the components used in solidelectro-optic displays, and the methods used to manufacture suchdisplays, are derived from technology used in liquid crystal displays(LCD's), which are of course also electro-optic displays, though using aliquid rather than a solid medium. For example, solid electro-opticdisplays may make use of an active matrix backplane comprising an arrayof transistors or diodes and a corresponding array of pixel electrodes,and a “continuous” front electrode (in the sense of an electrode whichextends over multiple pixels and typically the whole display) on atransparent substrate, these components being essentially the same as inLCD's. However, the methods used for assembling LCD's cannot be usedwith solid electro-optic displays. LCD's are normally assembled byforming the backplane and front electrode on separate glass substrates,then adhesively securing these components together leaving a smallaperture between them, placing the resultant assembly under vacuum, andimmersing the assembly in a bath of the liquid crystal, so that theliquid crystal flows through the aperture between the backplane and thefront electrode. Finally, with the liquid crystal in place, the apertureis sealed to provide the final display.

This LCD assembly process cannot readily be transferred to solidelectro-optic displays. Because the electro-optic material is solid, itmust be present between the backplane and the front electrode beforethese two integers are secured to each other. Furthermore, in contrastto a liquid crystal material, which is simply placed between the frontelectrode and the backplane without being attached to either, a solidelectro-optic medium normally needs to be secured to both; in most casesthe solid electro-optic medium is formed on the front electrode, sincethis is generally easier than forming the medium on thecircuitry-containing backplane, and the front electrode/electro-opticmedium combination is then laminated to the backplane, typically bycovering the entire surface of the electro-optic medium with an adhesiveand laminating under heat, pressure and possibly vacuum. Accordingly,most prior art methods for final lamination of solid electrophoreticdisplays are essentially batch methods in which (typically) theelectro-optic medium, a lamination adhesive and a backplane are broughttogether immediately prior to final assembly, and it is desirable toprovide methods better adapted for mass production.

The aforementioned U.S. Pat. No. 6,982,178 describes a method ofassembling a solid electro-optic display (including an encapsulatedelectrophoretic display) which is well adapted for mass production.Essentially, this patent describes a so-called “front plane laminate”(“FPL”) which comprises, in order, a light-transmissiveelectrically-conductive layer; a layer of a solid electro-optic mediumin electrical contact with the electrically-conductive layer; anadhesive layer; and a release sheet. Typically, the light-transmissiveelectrically-conductive layer will be carried on a light-transmissivesubstrate, which is preferably flexible, in the sense that the substratecan be manually wrapped around a drum (say) 10 inches (254 mm) indiameter without permanent deformation. The term “light-transmissive” isused in this patent and herein to mean that the layer thus designatedtransmits sufficient light to enable an observer, looking through thatlayer, to observe the change in display states of the electro-opticmedium, which will normally be viewed through theelectrically-conductive layer and adjacent substrate (if present); incases where the electro-optic medium displays a change in reflectivityat non-visible wavelengths, the term “light-transmissive” should ofcourse be interpreted to refer to transmission of the relevantnon-visible wavelengths. The substrate will typically be a polymericfilm, and will normally have a thickness in the range of about 1 toabout 25 mil (25 to 634 μm), preferably about 2 to about 10 mil (51 to254 μm). The electrically-conductive layer is conveniently a thin metalor metal oxide layer of, for example, aluminum or ITO, or may be aconductive polymer. Poly(ethylene terephthalate) (PET) films coated withaluminum or ITO are available commercially, for example as “aluminizedMylar” (“Mylar” is a Registered Trade Mark) from E.I. du Pont de Nemours& Company, Wilmington Del., and such commercial materials may be usedwith good results in the front plane laminate.

Assembly of an electro-optic display using such a front plane laminatemay be effected by removing the release sheet from the front planelaminate and contacting the adhesive layer with the backplane underconditions effective to cause the adhesive layer to adhere to thebackplane, thereby securing the adhesive layer, layer of electro-opticmedium and electrically-conductive layer to the backplane. This processis well-adapted to mass production since the front plane laminate may bemass produced, typically using roll-to-roll coating techniques, and thencut into pieces of any size needed for use with specific backplanes.

U.S. Pat. No. 7,561,324 describes a so-called “double release sheet”which is essentially a simplified version of the front plane laminate ofthe aforementioned U.S. Pat. No. 6,982,178. One form of the doublerelease sheet comprises a layer of a solid electro-optic mediumsandwiched between two adhesive layers, one or both of the adhesivelayers being covered by a release sheet. Another form of the doublerelease sheet comprises a layer of a solid electro-optic mediumsandwiched between two release sheets. Both forms of the double releasefilm are intended for use in a process generally similar to the processfor assembling an electro-optic display from a front plane laminatealready described, but involving two separate laminations; typically, ina first lamination the double release sheet is laminated to a frontelectrode to form a front sub-assembly, and then in a second laminationthe front sub-assembly is laminated to a backplane to form the finaldisplay, although the order of these two laminations could be reversedif desired.

U.S. Pat. No. 7,839,564 describes a so-called “inverted front planelaminate”, which is a variant of the front plane laminate described inthe aforementioned U.S. Pat. No. 6,982,178. This inverted front planelaminate comprises, in order, at least one of a light-transmissiveprotective layer and a light-transmissive electrically-conductive layer;an adhesive layer; a layer of a solid electro-optic medium; and arelease sheet. This inverted front plane laminate is used to form anelectro-optic display having a layer of lamination adhesive between theelectro-optic layer and the front electrode or front substrate; asecond, typically thin, layer of adhesive may or may not be presentbetween the electro-optic layer and a backplane. Such electro-opticdisplays can combine good resolution with good low temperatureperformance.

In a high-resolution display, each individual pixel must be addressablewithout interference from the addressing of adjacent pixels (whether ornot the electro-optic medium used is bistable). One way to achieve thisobjective is to provide an array of non-linear elements, such astransistors or diodes, wherein at least one non-linear element isassociated with each pixel, to produce an active matrix display, asmentioned above. An addressing (pixel) electrode, which addresses onepixel, is connected to an appropriate voltage source through itsassociated non-linear element. Conventionally, in high resolutionarrays, the pixels are arranged in a two-dimensional array of rows andcolumns, such that any specific pixel is uniquely defined by theintersection of one specified row and one specified column. The sourcesof all the transistors in each column are connected to a single columnelectrode, while the gates of all the transistors in each row areconnected to a single row electrode; the assignment of sources to rowsand gates to columns is conventional and could be reversed if desired.The row electrodes are connected to a row driver, which essentiallyensures that at any given moment only one row is selected, i.e., thatthere is applied to the selected row electrode a voltage such as toensure that all the transistors in the selected row are conductive,while there is applied to all other rows a voltage such as to ensurethat all the transistors in these non-selected rows remainnon-conductive. The column electrodes are connected to column drivers,which place upon the various column electrodes voltages selected todrive the pixels in the selected row to their desired optical states.(The aforementioned voltages are relative to a common front electrodewhich is conventionally provided on the opposed side of theelectro-optic medium from the non-linear array and extends across thewhole display.) After a pre-selected interval known as the “line addresstime” the selected row is deselected, the next row is selected, and thevoltages on the column drivers are changed so that the next line of thedisplay is written. This process is repeated so that the entire displayis written in a row-by-row manner.

In the discussion below, the term “waveform” will be used to denote theentire voltage against time curve used to effect the transition of apixel from one specific initial gray level to a specific final graylevel. Typically such a waveform will comprise a plurality of waveformelements; where these elements are essentially rectangular (i.e., wherea given element comprises application of a constant voltage for a periodof time); the elements may be called “pulses” or “drive pulses”. Theterm “drive scheme” denotes a set of waveforms sufficient to effect allpossible transitions between gray levels for a specific display. Adisplay may make use of more than one drive scheme; for example, U.S.Pat. No. 7,012,600 teaches that a drive scheme may need to be modifieddepending upon parameters such as the temperature of the display or thetime for which it has been in operation during its lifetime, and thus adisplay may be provided with a plurality of different drive schemes tobe used at differing temperature etc. A set of drive schemes used inthis manner may be referred to as “a set of related drive schemes.”

Prior art front electrodes for use with the electrophoretic and similarelectro-optic displays typically comprise a very thin (about 0.1 μm)layer of a ceramic, such as indium tin oxide or a similar mixed metaloxide (see the aforementioned U.S. Pat. No. 6,982,178). This thin layeris normally formed by sputtering the ceramic on to a polymer film,typically poly(ethylene terephthalate). Prior art rear (pixel)electrodes may be formed in a similar manner, or may be formed from thinmetal films; the front electrode must of course be light-transmissive toenable the electro-optic layer to be seen, whereas with a reflectiveelectro-optic layer, the rear electrodes can be opaque.

Although ceramic front electrodes have been in large scale commercialuse for a many years, they still suffer from a number of mechanical,optical and electrical problems. The tensions and temperatures usedduring lamination steps in the manufacture of the display may cause theceramic to crack and form discontinuities in conductivity, leading topoor or inconsistent switching of the display. These cracks are alsoareas of high water vapor transmission, which may cause local damage toa humidity-sensitive electro-optic medium (many of the aforementionedtypes of electro-optic media are sensitive to humidity). In colordisplay using color filter arrays (CFA's), it is desirable to reduceparallax problems by bringing the CFA as close as possible to theelectro-optic layer using a thin polymeric filmreducing the thickness ofthe PET substrate on which the ITO is coated. As the PET substrate ismade thinner the cracking issues associated with the ITO are accentuateddue to the higher thermal shrinkage of the thinner base.

As discussed in detail in the aforementioned U.S. Pat. No. 7,119,772, ithas been found desirable for at least some types of electro-opticdisplay that the drive scheme at each pixel location be DC balanced, inthe sense that, for any series of transitions beginning and ending atthe same gray level, the algebraic sum of the impulses applied duringthe series of transitions be bounded. It has been found that accuratelyDC-balanced waveforms (i.e., those in which the integral of currentagainst time for any particular pixel of the display is held to zeroover an extended period of operation of the display) are required topreserve image stability, maintain symmetrical switchingcharacteristics, and provide the maximum useful working lifetime incertain displays of the prior art.

It is in general preferred that all individual waveforms within a drivescheme be DC balanced, but in practice this has been difficult toachieve, so typical drive schemes have usually been a mixture of DCbalanced and DC imbalanced waveforms, even though the drive scheme as awhole has been DC balanced.

As discussed in the aforementioned U.S. Pat. No. 7,119,772, the extentto which DC-imbalanced driving affects an electrophoretic or otherelectro-optic display (presumably by polarization of certain displaycomponents, as discussed in more detail below) may be ascertained bymeasuring the open-circuit potential (hereinafter for convenience calledthe “remnant voltage”) of a particular region (say, a pixel) of thedisplay. When the remnant voltage of a pixel is zero, it is taken to beDC balanced. If its remnant voltage is positive, it is taken to be DCunbalanced in the positive direction. If its remnant voltage isnegative, it is taken to be DC unbalanced in the negative direction.Non-zero remnant voltages have been found to correlate with difficultiesin accurate gray level placement.

The degradation in display performance caused by development of remnantvoltage is generally reversible, either by storing the display withoutfurther switching or by switching appropriately to rebalance the DCimpulses. In cases where a prior art electrophoretic display is drivenwith extreme degrees of DC-imbalance, however, it is possible that theelectrodes may be irreversibly degraded, presumably by electrochemicalreactions that consume the electrode materials.

Although DC-balanced driving waveforms effectively protect againstdevelopment of remnant voltages and electrode degradation there areproblems associated with their use. Extra time must be allocated inorder to provide balancing impulses, sometimes resulting in update timesthat are two to three times longer than would be possible with aDC-imbalanced drive. In some electrophoretic compositions the timerequired for an optical transition from black to white is different fromthat required from white to black. In a DC-balanced waveform the longerof the two switching times must be used for both transitions. Inaddition, distracting optical transitions may be visible to the user ofthe display during DC-balanced updates.

As described for example in U.S. Pat. Nos. 6,724,519 and 7,564,614,corrosion inhibitors may be incorporated into electro-optic displays toprevent damage to electrodes from DC imbalances during driving of thedisplays. The present invention provides an alternative method ofpreventing damage to electrodes by incorporating into electro-opticdisplays materials which permit redox reactions to occur, thusprotecting the electrodes.

SUMMARY OF INVENTION

Accordingly, this invention provides an electro-optic display having aviewing surface through which a user views the display, a bistable,non-electrochromic electro-optic medium, and at least one electrodearranged to apply an electric field to the electro-optic medium, thedisplay further comprising at least 10 micromoles per square meter ofthe viewing surface (or alternatively of the area of the electro-opticmedium) of at least one compound (which may hereinafter be referred toas the “redox compound”) having an oxidation potential more negativethat about 150 mV with respect to a standard hydrogen electrode, asmeasured at pH 8.

The bistable, non-electrochromic electro-optic medium used in thedisplay of the present invention should normally be one which requires“charge injection” from its electrodes, i.e., it should normally be onewhich requires ionic conduction through one or more layers lying betweenthe electrodes. In the electro-optic display of the present invention,the electro-optic medium may be an electrophoretic medium comprising afluid and a plurality of electrically charged particles dispersed in thefluid. The redox compound may be in the form of a polymer provided in alayer disposed between one or more of the electrodes and the layer ofelectro-optic material.

The redox compound used in the electrophoretic display of the presentinvention may comprise one or more compounds of Formulae I-VIII below:

wherein, in Formulae I-III, R₁-R₁₅ may be substituted or unsubstitutedalkyl or aryl groups, or heteroatomic groups containing hetero atoms ofGroups V-VII of the periodic table;

wherein, in Formulae IV-VI, R₁₆-R₂₄ may be substituted or unsubstitutedalkyl or aryl groups, or heteroatomic groups containing hetero atoms ofGroups V-VII of the periodic table;

wherein, in Formula VII, R₂₅-R₂₈ may be substituted or unsubstitutedalkyl or aryl groups, or heteroatomic groups containing hetero atoms ofGroups V-VII of the periodic table;

R₂₉—SH  VIII

wherein, in Formula VIII, R₂₉ may be a substituted or unsubstitutedalkyl or aryl group or a heterocyclyl or heteroatomic group containinghetero atoms of Groups V-VII of the periodic table. Alternatively or inaddition, the redox compound may be any one or more of a phosphite salt,a sulfite salt; or a salt of titanium (III), vanadium (II), iron (II),cobalt (II) or copper (I).

Alternatively, the redox compound may be any one or more compoundsselected from the group consisting of hydroquinones, catechols,phenidone and substituted phenidone compounds, dihydropyridines andmetallocenes.

The displays of the present invention may be used in any application inwhich prior art electro-optic displays have been used. Thus, forexample, the present displays may be used in electronic book readers,portable computers, tablet computers, cellular telephones, smart cards,signs, watches, shelf labels, variable transmission windows and flashdrives.

This invention extends to a front plane laminate or inverted front planelaminate comprising a redox compound as defined above.

In prior displays, there is no provision for controlled electrochemicalreactions to occur at the electrode interfaces (i.e., to allowcontrolled charge injection from the electrodes). In contrast, indisplays of the present invention, redox compounds are incorporated inlayers adjacent the electrodes (or are incorporated into the displays inways which permit them to diffuse adjacent the electrodes), thuspermitting controlled electrochemical reactions to occur. Suchelectrochemical reactions may be reversible, partially reversible, orirreversible, and serve two purposes, namely to decrease the remnantvoltage observed with DC-imbalanced driving, and to protect theelectrode materials from irreversible degradation. With the addition ofthese redox compounds, DC-imbalanced driving can be effected withoutincurring an objectionable level of reversible or irreversible damage tothe display.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-section through an electrophoretic displayof the invention.

FIGS. 2 and 3 illustrate in simplified form the flux of chargedmaterials that may occur in a display 100 in response to an electricfield.

FIG. 4 is a graph showing the open circuit voltage (“remnant voltage”)following DC-balanced and DC-imbalanced driving for electrophoreticdisplays of the present invention and prior art control displays.

FIG. 5 is a graph shows the electrode degradation, as indicated byyellowing, for the same series of experiments as FIG. 4.

DETAILED DESCRIPTION

FIG. 1 shows a schematic cross-section through an encapsulatedelectrophoretic display 100, which may as described in, for example, theaforementioned U.S. Pat. No. 6,982,178. The display 100 comprises alight-transmissive substrate 102 that conveniently has the form of atransparent plastic film, such as a sheet of poly(ethyleneterephthalate) (PET) between 25 and 200 μm in thickness. Although notshown in FIG. 1, the substrate 102 (the upper surface of which, asillustrated in FIG. 1, forms the viewing surface of the display) maycomprise one or more additional layers, for example a protective layerto absorb ultra-violet radiation, barrier layers to prevent ingress ofoxygen or moisture into the display, and anti-reflection coatings toimprove the optical properties of the display.

The substrate 102 carries a thin, light-transmissive,electrically-conductive layer 104 that acts as the front electrode ofthe display. Layer 104 may comprise a continuous coating ofelectrically-conductive material with minimal intrinsic absorption ofelectromagnetic radiation in the visible spectral range such as indiumtin oxide (ITO), poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)(PEDOT:PSS), graphene or the like, or may be a discontinuous layer of amaterial such as silver (in the form of, for example, nanowires orprinted grids) or carbon (for example in nanotube form) that absorb orreflect visible light but are present at a surface coverage such thatthe layer as a whole is effectively transparent.

A layer (generally designated 108) of an electro-optic medium is inelectrical contact with the conductive layer 104 via a polymeric layeror layers 106 (which may be omitted). The electro-optic layer 108 ispreferably an opposite charge, dual particle encapsulatedelectrophoretic medium of the type described in U.S. Pat. No. 6,822,782,and may comprise a plurality of microcapsules, each of which maycomprise a capsule wall containing a hydrocarbon-based liquid in whichare suspended negatively charged white particles and positively chargedblack particles. The microcapsules may be retained within a polymericbinder. Upon application of an electrical field across the layer 108,the white particles move towards the positive electrode and the blackparticles move towards the negative electrode, so that the layer 108appears, to an observer viewing the display through the substrate 102,white or black depending upon whether the layer 104 is positive ornegative relative to the backplane electrode 112.

Alternatively, layer 108 may be fully encapsulated or comprise sealedmicro-cells or micro-cups, or may be non-encapsulated. Layer 108 maycomprise particles that move through a liquid solvent or a gas, orparticles that rotate within a solvent or a gas, or may modulate lightby displacement of a solvent, for example by electro-wetting.

As described for example in U.S. Pat. No. 6,982,178, the display 100further comprises a layer 110 of lamination adhesive coated over theelectro-optic layer 108. The lamination adhesive makes possible theconstruction of an electro-optic display by combining two subassemblies,namely a backplane 118 that comprises an array of pixel electrodes 112and an appropriate arrangement of conductors to connect the pixelelectrodes to drive circuitry, and a front plane 116 that comprises thesubstrate 102 bearing the transparent electrode 104, the electro-opticmaterial 108, the lamination adhesive 110 and optional additionalcomponents such as polymeric layer or layers 106. To form the finaldisplay, the front plane 116 is laminated to the backplane 118 usinglamination adhesive 110. The lamination adhesive may be cured thermallyor by actinic radiation (for example, by UV curing) or may be uncured.

Since the lamination adhesive 110 is in the electrical path separatingthe backplane electrodes 112 from the front plane electrode 104 itselectrical properties must be carefully tailored. As described in U.S.Pat. No. 7,012,735 the lamination adhesive may comprise, in addition toa polymeric material, an ionic dopant that may be an additive selectedfrom a salt, a polyelectrolyte, a polymer electrolyte, a solidelectrolyte, a conductive metal powder, a ferrofluid, a non-reactivesolvent, a conductive organic compound, and combinations thereof. Thevolume resistivities of encapsulated electrophoretic media are typicallyaround 10¹⁰ ohm cm, and the resistivities of other electro-optic mediaare usually of the same order of magnitude. Accordingly, the volumeresistivity of the lamination adhesive is normally around 10⁸ to 10¹²ohm cm at the operating temperature of the display, typically around 20°C.

Polymeric layer 106 may be a lamination adhesive layer with similarproperties to those described above with reference to laminationadhesive layer 110, except that, since polymeric layer 106 is adjacentto the non-pixelated, light-transmissive common electrode 104, itselectrical conductivity may be higher than that of lamination adhesivelayer 110, which is adjacent to the pixelated back plane electrodes 112and cannot be so conductive as to lead to significant currents flowingfrom one backplane electrode to its neighbors when they are held atdifferent potentials during switching of the display. When polymericlayer 106 is a lamination adhesive it may be used to affix electro-opticlayer 108 to electrode layer 104 during manufacture of the front planeas described in detail in the aforementioned U.S. Pat. No. 6,982,178.

FIG. 2 illustrates in simplified form the flux of charged materials thatmay occur in display 100 in response to an electric field applied bymeans of electrodes 104 and 112. For convenience, only polymeric layer106, electro-optic medium 108 and lamination adhesive 110 are shown,although, as will be clear to one of ordinary skill in the art, otherlayers are present in the display (for example, if the electro-opticmedium 108 is encapsulated, the capsule walls). Mobile charged speciesin each layer are shown generically as positively-charged species A, Cand E, and negatively-charged species B, D and F. Species A and B inpolymeric layer 106 could arise, for example, from an ionic dopant addedto enhance the conductivity of layer 106 or could be presentadventitiously in the materials used to form layer 106. For example, ifwater is present in layer 106 species A could correspond to a protonarising from ionization of the water. Species A may comprise more thanone mobile, cationic species; in this discussion A refers to any mobile,positively-charged species in layer 106. Likewise, species E and F referto positively- and negatively-charged mobile species in laminationadhesive layer 110. Species C and D refer to mobile, charged entities inthe electro-optic medium 108; such entities include charged pigmentparticles, whose motion changes the optical state of the display, andcharged species whose motion has no direct optical effect, such asmicellar charges that are well known in the art.

Charged species may cross the boundaries between the various layers ofthe display. This is shown schematically in FIG. 2, whereinpositively-charged species E is shown as crossing the boundary betweenthe lamination adhesive 110 and the electro-optic medium 108 andnegatively-charged species B is shown as crossing the boundary betweenlayers 106 and 108. If charged entities are displaced within theirrespective layers and cannot cross the boundaries between layers, chargewill accumulate at the impermeable boundary, being stored as if in anelectrolytic capacitor. After the applied electric field is removed andelectrodes 104 and 112 are grounded, discharge of the stored charge atboundaries within the display will occur, changing the electric fieldexperienced by the electro-optic layer 108 and potentially changing theoptical state of the display.

Even if mobile ionic charges can flow freely across the boundariesbetween the layers within the display (without accumulating at theinternal boundaries), there is still the difficulty that ionic speciescannot cross the boundaries between the interior layers of the displayand the electrodes 104 and 112. The only likely mechanism for chargetransfer across these boundaries is electron transfer, i.e.,reduction/oxidation chemical processes. If electron transfer betweenelectrodes 104 and 112 and the interior layers is blocked, ionic chargeswill inevitably build up at the boundaries between electrode 104 andlayer 106 and between layer 110 and electrode 112. If the display isdriven with a DC imbalanced drive scheme a substantial charge build-upat these locations may occur. Relaxation of the built-up charge when theelectrodes are brought to a common potential may lead to a flow ofcharge carriers through the electro-optic medium 108. This flow ofcharge carriers may lead to a change in the optical state of thedisplay.

In practical encapsulated or microcell displays of the prior art,parallel pathways for ionic conduction are provided by the walls of thecontainers (microcapsules or microcell walls) for the electro-opticmaterial 108, and thresholds of various kinds may be incorporated intoelectro-optic media so that return currents do not necessarily involvethe displacement of pigments. Such thresholds may be provided byincorporation of polymer into the fluid of the electrophoretic medium,as described for example in U.S. Pat. No. 7,170,670. Providing parallelconduction paths, thresholds, or other stabilization mechanisms alwaysresults in compromising the performance of the display, however, eitherin speed or in optical quality. It is therefore desirable to avoidaccumulation of charge at electrode boundaries.

More subtle problems may arise in the reproducibility of optical statesattained by the display in response to a given electrical impulse ifcharge displacements induced during the previous history of switching ofthe display are not completely nullified. Such problems are manifestedas “ghosting” in which traces of previously-displayed images are stillvisible many switching cycles after their original rendering. Asdiscussed in more detail below, charge displacements within layers 106and 110 may persist for considerable lengths of time.

As shown in FIG. 3, in the present invention charge accumulation atelectrode interfaces is reduced or eliminated by electrochemicaloxidation/reduction reactions involving electron transfer from theelectrodes to materials within layers 106 and 110. At the cathode anelectron is transferred to reduce a component in the adjacent layer(reduction of neutral species G to provide anion G− is illustrated),while at the anode an electron is transferred to the electrode resultingin oxidation of a component in the adjacent layer (electron transferfrom neutral species J to produce cation J+ is shown). The injectedcharges are of the opposite sign to the charges displaced during initialpolarization of the display, and thus the charge buildup at theelectrode interfaces is reduced or eliminated.

When a potential is applied to a device such as the display 100illustrated in FIG. 3, migration of ions towards the electrodes leads toaccumulation of charge in thin diffuse layers near the electrodes, thethickness of these layers being of the order of the Debye screeninglength, as is well known in the electrochemical art. Within thesediffuse layers the gradients in electrical potential are very steep.After a certain time, the potential gradient becomes sufficient to causeelectron transfer reactions. The ease of electron transfer is determinedby, among other things, the redox potentials of the materials present inthe vicinity of the electrodes and their concentrations.

Chemical materials that may be used as redox compounds in the displaysof the present invention will now be discussed in detail. Such materialsfind use in other electrochemical devices, for example, batteries,photovoltaic cells, fuel cells, electrochromic display elements, and thelike.

In general it is preferred that a redox compound used in the display ofthe present invention have a reduction potential (of the oxidized form)that is not greater than 1.0 V relative to a standard hydrogenelectrode, that the redox material contain at least one carbon atom, andthat its molecular weight not exceed 1000 Daltons. Particularlypreferred redox compounds have a reduction potential that is not greaterthan 0.2 V relative to a standard hydrogen electrode, contain at leastone carbon atom, and have a molecular weight not exceeding 1000 Daltons.Materials meeting these criteria have been found to be compatible withtypical lamination adhesives (i.e., layers 106 and 110 in FIG. 1), todiffuse through the adhesive sufficiently quickly to participate inelectrochemical reactions at the electrodes, and to be sufficientlyeasily oxidized that remnant voltages observed following extendedDC-imbalanced driving do not exceed about 1 V.

It is believed (though the invention is in no way limited by thisbelief) that, in the absence of a redox compound in accordance with thepresent invention, oxidation of water to form oxygen (a half-cellreaction with a standard reduction potential of more than 1 V) mayoccur. Because the redox compounds used in the present invention aremore easily reduced than water, when such redox compounds are presentless ionic polarization is required to produce a sufficiently steeppotential gradient in the electrode double layer for electron transferto take place, and the remnant voltage experienced by the electro-opticmaterial is consequently lower.

In some preferred embodiments of the present invention a mixture ofoxidized and reduced forms of a redox material is used; in otherembodiments it may be sufficient to use only the reduced form, in whichcase it is thought that the corresponding reduction reaction at thesecond electrode may involve an adventitious material present in thedisplay, possibly a proton arising from water commonly present inpolymeric materials.

Specific preferred redox compounds useful in the present invention willnow be described. Compounds of Formulae I-III above comprise onepreferred class of redox compounds. The oxidation of such materialsinvolves two electrons and results in the liberation of two protons andthe formation of a benzoquinone (although, as is well known in thechemical art, single electron reactions resulting in semiquinonestructures are also possible). Substituents R₁-R₁₅ may be substituted orunsubstituted alkyl or aryl groups, or heteroatomic groups containinghetero atoms of Groups V-VII of the periodic table (hereinafter forconvenience abbreviated as “heteroatomic groups of Groups V-VII”).Substituents R₁ and R₂ (taken together), and/or R₃ and R₄, and/or R₅ andR₆, and/or R₇ and R₈, and/or R₁₀ and R₁₁, and/or R₁₂ and R₁₃, may form aring. Particularly preferred materials of this class includetetramethylhydroquinone, trimethylhydroquinone, and 2,4- and2,5-di-tert-butyl hydroquinone. Particularly preferred materials of thisclass have substituents at every position R₁-R₄, or R₅-R₈, or R₁₀-R₁₃which are preferably alkyl or aryl substituents. Such materials are lessprone to the formation of colored byproducts.

Compounds of Formulae IV-VI above comprise a second preferred class ofredox compounds for use in the present invention, this class comprisingan unsaturated 1,2-dihydroxy (or a nitrogen-containing equivalent)substructure. Although a cis isomer is illustrated, trans isomers mayalso be used. Substituents R₁₆-R₂₂ may be substituted or unsubstitutedalkyl, or aryl groups, or heteroatomic groups of Groups V-VII.Substituents R₁₆ and R₁₇ (taken together), and/or R₁₈ and R₁₉, and/orR₂₁ and R₂₂ may form a ring. Particularly preferred materials of thisclass include ascorbic acid and catechols such as4,5-di-tert-butyl-1,2-dihydroxybenzene.

Compounds of Formula VII comprise a third preferred class of redoxcompounds for use in the present invention, this class comprising ahydrazine moiety. In this class, a nitrogen atom is oxidized to aradical cation that may react further (for example, by radical coupling)to eventually liberate a proton. Substituents R₂₅-R₂₈ may be substitutedor unsubstituted alkyl or aryl groups, or heteroatomic groups of GroupsV-VII. Substituents R₂₅ and R₂₆ (taken together), and/or R₂₇ and R₂₈ mayform a ring. Particularly preferred materials of this class includephenidone and related materials.

Compounds of Formula VIII comprise a fourth preferred class of redoxcompounds, in which two sulfur atoms are oxidized to radical cationsthat couple to form a disulfide bond and liberate two protons.Substituent R₂₉ may be a substituted or unsubstituted alkyl or arylgroup, or a heterocyclyl or heteroatomic group of Groups V-VII. Aparticularly preferred material of this class is5-mercapto-1-methyltetrazole.

In an electrophoretic display, it is necessary to drive the top planeelectrode to either a positive or a negative potential relative to thebackplane electrodes without a bias in conductivity. This requirementprecludes the use, in the present invention, of redox cascades ofdecreasing redox potential such as are commonly used in the art to makediodes (such as light-emitting diodes).

As mentioned above, the use of a redox compound in accordance with thepresent invention is intended to control the build-up of charge in thediffuse layers adjacent to the electrodes. Such a build up of charge istypically a reversible process. The present invention also seeks tocontrol the nature of the Faradaic reactions that occur at the electrodeinterfaces so as to enable DC imbalanced driving of a display withoutincurring irreversible electrode damage. Without the use of controlledredox compounds in accordance with the invention, unwanted Faradaicreactions may occur, such as electrolysis of water, leading to theformation of byproducts such as hydrogen and oxygen gas. Even worse, theelectrode materials themselves may participate in redox reactions. Manyconventional transparent electrode materials are prone to reductionreactions; for example, indium tin oxide (ITO) may be irreversiblyreduced to metallic tin (or indium), leading at first to discoloration(yellowing) of the transparent electrodes and eventually to completefailure. PEDOT:PSS may lose its conductivity when reduced. Othermaterials used as transparent electrodes are prone to oxidation; forexample, silver metal nanowires or grids may be readily oxidized tosilver cations. The present invention seeks to introduce competitiveredox chemistry to allow Faradaic reactions to occur at the electrodeinterfaces without degradation of the electrodes.

Depending upon the exact materials present in the display, the redoxcompounds used in the present invention may be added to the display ineither their reduced or their oxidized form, or a mixture of the twoforms. Also, it may in some cases be desirable to add the oxidized formof one redox compound and the reduced form of a different redoxcompound. Oxidized forms of the redox compounds are incorporated toprevent undesired reduction of electrodes such as indium tin oxide. Insome cases where oxidized and reduced forms of different redox compoundsare used, it may be desirable that the standard reduction potential ofthe oxidized form be less positive than that of the oxidized form of theredox compound added in its reduced form; if this criterion is met, theoxidized and reduced forms will not react with each other.

Specific preferred oxidized forms of redox compounds for use in thepresent invention include benzoquinone materials of Formula V whereinsubstituents R₁₁-R₁₄ are substituted or unsubstituted alkyl, aryl, orheteroatomic groups of Groups V-VII of the periodic table. SubstituentsR₁₁ and R₁₂ taken together, or substituents R₁₃ and R₁₄ taken together,may form a ring. Particularly preferred materials of this type includetetramethylbenzoquinone (also known as duroquinone),trimethylbenzoquinone, and 2,4- and 2,5-di-tert-butyl benzoquinone.Particularly preferred materials of this class have each of thesubstituents R₁₁-R₁₄ as alkyl or aryl substituents. Such materials areless prone to the formation of colored by-products.

Preferably, the reduced form of the redox compound used in the presentinvention should be easily oxidized but it must not, of course, reactwith oxygen in the air. In addition, the reduced form should have arelatively low molecular weight such that it can diffuse to theelectrode sufficiently quickly to undergo oxidation during operation ofthe display; molecular weights less than 1000 Daltons are preferred inthe present invention. The concentration of the reduced form should besuch that it is not exhausted during the lifetime of the display. Sinceelectrophoretic displays are driven with driving pulses of bothpolarities, redox compounds capable of reversible redox reactions arepreferred to those undergoing irreversible reactions, but the ability toundergo reversible redox reactions is not an absolute requirement of theinvention. It has been found that concentrations of the reduced form ofthe invention in excess of 10 mmole per square meter of the viewingsurface (or of the electro-optic medium) of the display are required inorder to avoid premature exhaustion of the redox compound.

The oxidized form of a redox compound used in the present inventionshould be more easily reduced than the ITO electrode; providing acompeting pathway for reduction serves to protect the ITO electrode fromirreversible electrochemical degradation. The oxidized form should not,however, be so easily reduced that it thermally oxidizes a reduced formunless such an oxidation produces a new reduced form (i.e., the reducedform corresponding to the oxidized form). For example, a preferredreduced form is phenidone and a preferred oxidized form istetramethylbenzoquinone. Phenidone might be oxidized bytetramethylbenzoquinone, and one product of this reaction would betetramethylhydroquinone, which is itself a reduced form useful in thepresent invention. Thus, if tetramethylbenzoquinone is incorporated intoa display in molar excess over phenidone, after complete thermalreaction a mixture of tetramethylbenzoquinone (an oxidized form) andtetramethylhydroquinone (a reduced form) would be present.

EXAMPLES

The following Examples are given, though by way of illustration only, toshow details of specific materials and processes useful in the presentinvention.

Example 1

This Example illustrates the reduction in remnant voltage and electrodedamage following DC imbalanced driving of a display of the presentinvention, as compared with a control display lacking a redox compound.

Experimental displays were prepared as follows:

Part A: Preparation of a Solution of Redox Compounds

An oxidized form of a redox compound (tetramethylbenzoquinone, 370 mg,2.3 mmole) and a reduced form of a redox compound (phenidone, 190 mg,1.2 mmole) were added to isopropanol (10 g) and the mixture wassonicated at 35° C. for 15 minutes.

Part B: Preparation of a Lamination Layer (Corresponding to Layer 106 inFIG. 1)

The redox compound solution prepared in Part A above (7.8 g) was addedto 92.2 g of a 35 percent by weight aqueous dispersion of a polyurethanelatex of the type described in U.S. Pat. No. 7,342,068 and the mixturewas homogenized on a roll mill. The resultant mixture was coated on to apoly(ethylene terephthalate) (PET) film base of 4 mil thickness bearinga coating of indium tin oxide (no) to give a wet thickness ofapproximately 100 μm and the coating was air dried at 140° F. (60° C.).

Part C: Preparation of a Lamination Layer (Corresponding to Layer 110 inFIG. 1)

The redox compound solution prepared in Part A above (1.4 g) was addedto 78.6 g of an 8% solution in isopropanol of a polyurethane of the typedescribed in U.S. Pat. No. 7,342,068. The solution thus prepared wascoated on to a metalized release sheet to produce a wet layer with athickness of approximately 80 μm and air dried at 25° C. giving a finalthickness of approximately 5 μm.

Part D: Preparation of Displays (as Illustrated in FIG. 1)

The dried film prepared in Part B above was laminated using a hot rolllaminator at 250° F., 0.5 ft/minute and 62 psi (121° C., 2.5 mm/sec andapproximately 0.45 MPa) to a coating of microcapsules containing anelectrophoretic fluid on a release film prepared as described in U.S.Pat. No. 7,561,324. The release film was removed and the resultantassembly was laminated, together with the film prepared in Part C above,using the same laminator at 200° F., 0.5 ft/minute and 62 psi. (93° C.,2.5 mm/sec and approximately 0.45 MPa) The metalized release sheet wasthen removed and the resultant structure was laminated either to a sheetof PET bearing a conductive carbon coating or to a sheet of glassbearing an ITO coating (to form the electrode 112 and rear substrate 114shown in FIG. 1) to form the experimental displays. These displays wereconditioned at 50° C./50% relative humidity (RH) for 5 days.

The experimental displays thus made with ITO/glass backplanes werecompared with similar displays lacking the redox compounds used in thepresent invention. The displays were driven with repeated iterations ofthe DC-imbalanced and DC-balanced waveforms shown in Table 1 below formany hours, after which the open circuit voltage was measured byapplying +1 V across the sample's terminals and measuring the current,then applying −1 V and again measuring the current. Linear interpolationbetween the two current measurements was used to find the voltage atwhich the current would have been zero and this voltage was reported asthe open circuit voltage. To reduce the measurement of capacitiveeffects, the voltages were applied for 1 second but the average currentwas measured only over the last 200 ms of this period.

TABLE 1 Time Duration Voltage Voltage (sec) (sec) (imbalanced)(balanced) 0 0.24 −15 15 0.24 1.00 0 0 1.24 0.24 15 15 1.48 1.00 0 02.48 0.24 15 15 2.72 0.11 0 0 2.83 0.24 15 15 3.07 0.11 0 0 3.18 0.24 1515 3.42 0.11 0 0 3.53 0.24 15 15 3.77 0.11 0 0 3.88 0.24 15 15 4.12 0.110 0 4.23 0.24 15 15 4.47 0.11 0 0 4.58 0.24 −15 15 4.82 0.11 0 0 4.930.24 15 15 5.17 0.11 0 0 5.28 0.24 −15 15 5.52 0.11 0 0 5.63 0.24 15 155.87 0.11 0 0 5.98 0.24 −15 15 6.22 0.11 0 0 6.33 0.24 15 15 6.57 0.11 00

FIG. 4 shows the open circuit voltages for a display comprising theredox compounds as compared with a control lacking these compounds. Bothdisplays, when driven with a DC-balanced waveform, exhibited only smallchanges in the measured open-circuit voltage. The display of theinvention exhibited less than half the open-circuit voltage build-up ofthe control display after prolonged driving with the DC-imbalancedwaveform.

FIG. 5 shows the b* value of the white state of the displays driven asdescribed above. As is well known in the art, the b* value in theconventional CIE L*a*b* color space is a measure of the yellowness of asample. The development of a yellow color in an ITO electrode is knownto correlate with irreversible reduction of the ITO. Both displays, whendriven with a DC-balanced waveform, exhibited only small changes in themeasured b* value of the white state. The display of the inventionexhibited about one fifth of the change in b* of the control displayafter prolonged driving with the DC-imbalanced waveform.

Example 2

This Example illustrates the improved stability of white and dark statesfollowing a DC-imbalanced drive achieved by a display of the presentinvention as compared with a control lacking the redox compounds.

Experimental displays produced in Example 1 above with PET/carbonbackplanes were compared with similar control displays lacking the redoxcompounds. The displays were driven to the white state in a highlyimbalanced manner by applying the following waveform: three iterationsof ten 250 ms pulses of +15 V (top plane) each, separated by 1 second at0 V, with 5 seconds at 0 V after the tenth pulse, followed by 25 secondsat 0 V. The difference between the white state measured at the end ofthe final 15 V pulse and that measured at the end of the whole waveformwas recorded. Similarly, the displays were driven to the dark state withthe same waveform except with −15 V pulses and the difference betweenthe dark state measured at the end of the final −15 V pulse and thatmeasured at the end of the whole waveform was recorded. The results areshown in Table 2 below.

TABLE 2 White State change (L*) Dark State change (L*) Invention −4.53.6 Control −11.8 11.2

From Table 2, it will be seen that the display of the present inventionproduced changes in both white and dark states which were much lowerthan those of the control display, this illustrating the reducedpolarization of the display produced by the DC imbalanced waveform.

From the foregoing, it will be seen that the addition of redox compoundsto electro-optic displays in accordance with the present invention canprovide substantially reduced remnant voltages are protection againstelectrochemical degradation of electrodes.

It will be apparent to those skilled in the art that numerous changesand modifications can be made in the specific embodiments of theinvention described above without departing from the scope of theinvention. Accordingly, the whole of the foregoing description is to beinterpreted in an illustrative and not in a limitative sense.

1. An electro-optic display comprising: a viewing surface through whicha user views the display; a layer of bistable, non-electrochromicelectro-optic medium; an electrode arranged to apply an electric fieldto the electro-optic medium; and a polymer layer interposed between theelectro-optic medium and the electrode, the polymer layer comprising atleast 10 micromoles per square meter of the viewing surface of a sulfitesalt or a salt of titanium (III), vanadium (II), iron (II), cobalt (II)or copper (I), wherein the sulfite salt or the salt of titanium (III),vanadium (II), iron (II), cobalt (II) or copper (I) has an oxidationpotential more negative than 150 mV with respect to a standard hydrogenelectrode, as measured at pH
 8. 2. The electro-optic display of claim 1,wherein the sulfite salt or the salt of titanium (III), vanadium (II),iron (II), cobalt (II) or copper (I) has a reduction potential (of theoxidized form) that is not greater than 1.0 V relative to a standardhydrogen electrode.
 3. The electro-optic display of claim 1, wherein thepolymer layer comprises both the oxidized and the reduced forms of thesulfite salt or the salt of titanium (III), vanadium (II), iron (II),cobalt (II) or copper (I).
 4. The electro-optic display of claim 1,where in the electro-optic medium is an electrophoretic mediumcomprising a fluid and a plurality of electrically charged particlesdispersed in the fluid.
 5. The electro-optic display of claim 4, whereinthe electrically charged particles and the fluid are confined within aplurality of capsules or microcells.
 6. The electro-optic display ofclaim 4, wherein the electrically charged particles and the fluid arepresent as a plurality of discrete droplets surrounded by a continuousphase comprising a polymeric material.
 7. An electro-optic displaycomprising: a viewing surface through which a user views the display; alayer of bistable, non-electrochromic electro-optic medium; an electrodearranged to apply an electric field to the electro-optic medium; and apolymer layer interposed between the electro-optic medium and theelectrode, the polymer layer comprising at least 10 micromoles persquare meter of the viewing surface of a hydroquinone, a catechol, adihydropyridine or a metallocene, and wherein the hydroquinone, thecatechol, the dihydropyridine or the metallocene has an oxidationpotential more negative than 150 mV with respect to a standard hydrogenelectrode, as measured at pH
 8. 8. The electro-optic display of claim 7,wherein the hydroquinone, the catechol, the dihydropyridine or themetallocene has a reduction potential (of the oxidized form) that is notgreater than 1.0 V relative to a standard hydrogen electrode.
 9. Theelectro-optic display of claim 7, wherein the polymer layer comprisesboth the oxidized and the reduced forms of the hydroquinone, thecatechol, the dihydropyridine or the metallocene.
 10. The electro-opticdisplay of claim 7, where in the electro-optic medium is anelectrophoretic medium comprising a fluid and a plurality ofelectrically charged particles dispersed in the fluid.
 11. Theelectro-optic display of claim 10, wherein the electrically chargedparticles and the fluid are confined within a plurality of capsules ormicrocells.
 12. The electro-optic display of claim 10, wherein theelectrically charged particles and the fluid are present as a pluralityof discrete droplets surrounded by a continuous phase comprising apolymeric material.